I learned this the hard way. Two months ago I had to take my van in for a brake job. The shop was 7 miles from my house so I tossed my Trek 7200 in the back so I could ride it home.

Everything was OK for about two miles and then the drive side pedal felt funny, like I couldn't get a good foot position on it. I figured it was because I was wearing a new pair of shoes and the soles just felt different on the pedal. Another mile later I knew that something was wrong as the whole pedal felt funny by then.

Pulled over and found out the pedal was really loose and wobbly and had almost completely stripped out the crank arm. It was only catching about 1 to 1 1/2 of the pedal threads on the end of the pedal. I carry a tire lever set, tube, small multi tool kit, and a cartridge inflater, but no pedal wrench. I used the tire levers to grip the pedal flats and snug it up a bit.

Continued my ride home with the pedal getting worse with every stroke and not wanting to ride with one pedal or walk a broken bike for 4 miles. Got pretty scary for about a 1/4 mile where I had to ride out in traffic on the way home but I made it.

The pedal fell out when I coasted into my driveway. The driveside crank arm (Shimano M-191) was junk, had to replace it and get new pedals to the tune of about 40 bucks.

Makes me mad at myself as I should have checked the pedals during the pre-ride check. Trust me, I won't make that mistake again and now have a small pedal wrench in my kit too, just in case.

If that pedal would have popped out when I was up off the saddle gaining speed to keep up with traffic ..... well lets just say, it wouldn't be a very good thing